7. Mass Transport

Question 1

Describe the primary structure of haemoglobin

Answer 1

Sequence of amino acids in the 4 polypeptide chains

Question 2

Describe the secondary structure of haemoglobin

Answer 2

The polypeptide chains are coiled into a helix

Question 3

Describe the tertiary structure of haemoglobin

Answer 3

Each polypeptide chain is folded into a precise shape

Question 4

Describe the quaternary structure of haemoglobin

Answer 4

All 4 polypeptides are linked together to form an almost spherical molecule. Each polypeptide is associated with a haem group- contains a ferrous (Fe2+) ion.
Each Fe2+ ion can combine with a single oxygen (O2) molecule, a total of 4 O2 molecules can be carried by a single haemoglobin.

Question 5

Define ‘loading’ / ‘associating’

Answer 5

The process by which haemoglobin binds with oxygen. In humans this takes place in the lungs.

Question 6

Define ‘unloading’ / ‘dissociating’

Answer 6

The process by which haemoglobin releases its oxygen. In humans this takes place in the tissues.

Question 7

What does it mean if haemoglobin has a ‘high affinity’ for oxygen?

Answer 7

Haemoglobins take up oxygen more easily, but release it less easily.

Question 8

What does it mean if haemoglobin has a ‘low affinity’ for oxygen?

Answer 8

Haemoglobins take up oxygen less easily, but release it more easily.

Question 9

How does haemoglobin readily associate with oxygen at the lungs and readily dissociate from oxygen at the tissues?

Answer 9

Haemoglobin changes its affinity for oxygen under different conditions because its shape changes in the presence of certain substances, e.g CO2. In the presence of CO2, the new shape of haemoglobin binds more loosely to oxygen, so it loses the oxygen.

Question 10

Why do different haemoglobins have different affinities for oxygen?

Answer 10

Because of the shape of the molecule. Each species produces haemoglobin with a slightly different amino acid sequence, and therefore a different tertiary and quaternary sequence, hence different oxygen binding properties.

Question 11

Why do different organisms have different haemoglobins?

Answer 11

Different organisms take up and release oxygen differently, therefore the shape of their haemoglobin needed to differ.

Question 12

When the body is at rest, only one of the four oxygen molecules carried by haemoglobin is normally released into the tissues. Suggest why this could be an advantage when the organism becomes more active

Answer 12

If all the oxygen molecules were released, there would be none in reserve to supply tissues when they were more active.

Question 13

Carbon monoxide occurs in car exhaust fumes. It binds permanently to haemoglobin in the presence of oxygen. Suggest a reason why a person breathing in car exhaust fumes might lose consciousness.

Answer 13

Carbon monoxide will gradually occupy all the sites on haemoglobin instead of oxygen. No oxygen will be carried to tissues such as the brain. These will cease to respire and to function, making the person lose consciousness.

Question 14

Define an ‘oxygen dissociation curve’

Answer 14

The graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen.

Question 15

Explain the shape of an oxygen dissociation curve

Answer 15

• At low oxygen concentrations, little oxygen binds to haemoglobin- curve initially shallow.
• The binding of the first oxygen causes the quaternary structure of the haemoglobin to change shape, inducing the subunits to bind to an oxygen molecule.
• It takes a smaller increase of partial pressure of oxygen to bind the 2nd oxygen than the 1st and so on- ‘partial cooperativity’. Curve gradient steepens.
• After binding to the 3rd oxygen, it is hard for the haemoglobin to bind to the 4th oxygen because the majority of binding sites are occupied so its harder for the oxygen to find a free site. Curve gradient reduces and graph flattens off.

Question 16

What does it mean if the oxygen dissociation curve is further to the left?

Answer 16

The further left, the greater the affinity of haemoglobin for oxygen (so loads readily, unloads less easily).

Question 17

What does it mean if the oxygen dissociation curve is further to the right?

Answer 17

The further right, the lower the affinity of haemoglobin for oxygen (so loads oxygen less readily, unloads more easily).

Question 18

At the lungs, how does concentration of carbon dioxide affect the affinity for oxygen and the dissociation curve?

Answer 18

At the lungs, the concentration of carbon dioxide is low because it diffuses across the exchange surface and is excreted. The affinity for oxygen is increased, due to increased pH which changes the shape of haemoglobin, and because the oxygen concentration is high in the lungs, oxygen is readily loaded by haemoglobin. The reduced carbon dioxide concentration shifts the oxygen dissociation curve to the left.

Question 19

At the tissues, how does concentration of carbon dioxide affect the affinity for oxygen and the dissociation curve?
(Boer effect)

Answer 19

In respiring tissues, the concentration of carbon dioxide is high. The affinity of haemoglobin for oxygen is low, and the concentration of oxygen is low, so affinity for oxygen decreases, due to lowering pH which changes shape of haemoglobin. Oxygen is readily unloaded from haemoglobin into muscle cells. Increased carbon dioxide concentration shifts the curve to the right.

Question 20

A rise in temperature shifts the oxygen dissociation curve to the right. Suggest how this enables an exercising muscle to work more efficiently.

Answer 20

Exercising muscles release heat, shifting the curve to the right and causing the haemoglobin to release more oxygen to fuel the muscular activity.

Question 21

Haemoglobin usually loads oxygen less readily when the concentration of carbon dioxide is high (Boer effect). Haemoglobin of lugworms don’t exhibit this effect. Explain how to do so would be harmful.

Answer 21

Respiration produces carbon dioxide. This builds up in the burrow when the tide is out. If lugworm haemoglobin exhibited the Bohr effect, it would not be able absorb oxygen when it was present in low concentrations in the burrow.

Question 22

In terms of obtaining oxygen, explain why lugworms are not found higher up the seashore.

Answer 22

The higher part of the beach is uncovered by the tide for a much longer time than the lower part. During this longer period all the oxygen in the burrow would be used up and the lugworm might die before the tide brings in the next supply of oxygen

Question 23

Name common features of transport systems

Answer 23

• A suitable medium to carry materials, e.g blood
• A form of mass transport in which the transport medium is moved in bulk over large distances, quicker than diffusion.
• Closed system of tubular vessels containing the transport medium and forms a branching network.
• Mechanism for moving the transport medium within vessels, requires pressure difference.

Question 24

Why is the blood passed twice through the heart in one circuit of the human closed double circulatory system?

Answer 24

Pressure is reduced when blood passes through the lungs. If passed immediately to the rest of the body circulation would be slow. Blood is returned to the heart to boost its pressure so all substances are circulated to the body tissues quickly.

Question 25

Name the blood vessel that joins the right ventricle of the heart to the capillaries of the lungs.

Answer 25

Pulmonary artery

Question 26

Name the blood vessel that carries oxygenated blood away from the heart.

Answer 26

Aorta

Question 27

Name the blood vessel that carries deoxygenated blood away from the kidney.

Answer 27

Renal vein

Question 28

Name the first main blood vessel that an oxygen molecule reaches after being absorbed from an alveolus

Answer 28

Pulmonary vein

Question 29

Name the blood vessel that has the highest blood pressure.

Answer 29

Aorta

Question 30

State two factors that make it more likely that an organism will have a circulatory pump such as the heart.

Answer 30

• Low surface area to volume ratio

- High metabolic rate.

Question 31

What is the cardiac cycle?

Answer 31

A continuous series of events which make up a single heartbeat.

Question 32

Describe the diastole phase(relaxation of the heart)

Answer 32

Blood returns to the atria via the pulmonary vein & the vena carva. As the atria fill, pressure rises. When the pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles. The relaxation of the ventricle walls causes them to recoil and reduces the pressure within the ventricle. This causes the pressure to be lower than that in the aorta and the pulmonary artery close, accompanied by a ‘dub’ sound of heartbeat.

Question 33

Describe atrial systole phase (contraction of the atria)

Answer 33

The contraction of the atrial walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria. Ventricle walls remain relaxed.

Question 34

Describe the ventricular systole phase (contraction of the ventricles)

Answer 34

After the ventricles fill with blood, their walls contract simultaneously. This increases the blood pressure within them, closing the atrioventricular valves, preventing back-flow of blood into the atria- ‘lub’ sound of heartbeat. With valves closed, pressure rises in ventricles further. Once it exceeds that in the aorta & pulmonary artery, blood is forced from the ventricles into these vessels. Ventricles thick muscular walls allow them to contract forcefully, creating a high pressure to pump blood around the body.

Question 35

Explain why the muscular wall of the left ventricle is thicker than that of the right ventricle

Answer 35

The left ventricle is thicker to contract at higher pressure as blood is pumped around the body. The right ventricle is thinner as the pressure is lower because it only pumps blood to the lungs.

Question 36

Where is the atrioventricular valves located?

Answer 36

Between the left atrium and ventricle and right atrium and ventrcicle

Question 37

What is the function of the atrioventricular valves?

Answer 37

Prevent backflow of blood when contraction of the ventricles means that ventricular pressure exceeds atrial pressure. Closure of these valves ensures that, when the ventricles contract, blood within them moves to the aorta and pulmonary artery, rather than back to the atria.

Question 38

Where are semi-lunar valves located?

Answer 38

The aorta and pulmonary artery

Question 39

What is the function of the semi-lunar valves?

Answer 39

Prevent backflow of blood into ventricles when the pressure in these vessels exceeds that of the ventricles. This arises when the elastic walls of the vessels recoil, increasing the pressure within them and when the ventricle walls relax reducing the pressure within the ventricles.

Question 40

Where are the pocket valves located?

Answer 40

In the veins that occur throughout the venous system

Question 41

What is the function of the pocket valves?

Answer 41

Ensures that, when veins are squeezed, blood flows back towards the heart rather than away from it.

Question 42

Name features of valves

Answer 42

Valves are made up of a number of flaps of tough, flexible, fibrous tissue, which are cusp shaped. Under pressure, the convex side of the cusps part to let blood pass through. When pressure is greater on the concave side of the cusps, blood collects within the cusps, pushing them together to prevent backflow of blood.

Question 43

Name 2 variables the cardiac output depends upon

Answer 43

-The heart rate
-The stroke volume
(cardiac output=heart rate x stroke volume)

Question 44

What is the function of arteries?

Answer 44

Arteries carry blood away from the heart and into arterioles

Question 45

How is artery structure adapted to its function?

Answer 45

• Thick muscle layer compared to veins- smaller arteries can be constricted/dilated to control bloodflow.
• Thick elastic layer compared to veins- blood pressure in arteries is high to reach the body. Elastic wall stretched at systole, and springs back at diastole.
• Overall wall thickness is great- resists vessel bursting under pressure.
• No valves- blood under constant high pressure, doesn’t tend to flow backwards.

Question 46

What is the function of arterioles?

Answer 46

Arterioles are smaller arteries that control bloodflow from arteries to capillaries.

Question 47

How is arteriole structure adapted to its function?

Answer 47

• Muscle layer thicker than arteries- contraction of this muscle layer allows constriction of the lumen of the arteriole, restricts bloodflow & controls movement into the capillaries.
• The elastic layer is thinner than arteries- blood pressure is lower

Question 48

What is the function of veins?

Answer 48

Veins carry blood from capillaries back to the heart

Question 49

How is vein structure adapted to its function?

Answer 49

• Muscle layer thin- veins carry blood away from tissues, so their constriction/dilation can’t control bloodflow to tissues.
• Elastic layer thin- low blood pressure.
• Overall thickness of wall is small- pressure too low for risk of bursting. Allows them to be flattened easily.
• Valves at intervals throughout-ensure no backflow of blood, due to low pressure. When body muscles contract, veins contract, pressurising blood. Valves ensure blood only flows towards the heart.

Question 50

What is the function of capillaries?

Answer 50

Capillaries are tiny vessels that link arterioles to veins.

Question 51

How is capillary structure adapted to its function?

Answer 51

• Walls consist mostly of the lining layer, extremely thin, diffusion distance short- rapid exchange between blood & cells.
• Numerous and highly branched- large SA for exchange.
• Narrow diameter- permeate tissues, no cell far from capillary, short diffusion distance.
• Narrow lumen- red blood cells squeezed flat against side of capillary. Brings them closer to the cells to which they supply oxygen, reducing diffusion distance.
• Spaces between endothelial cells that allow white blood cells to escape to respond to infection.

Question 52

Define ‘tissue fluid’

Answer 52

Fluid that surrounds the cells of the body, Composition similar to blood plasma, except it lacks proteins. Supplies cells with nutrients, e.g. glucose, amino acids, fatty acids, ions & oxygen. Removes waste from cells, e.g carbon dioxide.

Question 53

Describe the formation of tissue fluid

Answer 53

• Pumping of blood by the heart creates hydrostatic pressure, at the arterial end of the capillaries, causing tissue fluid to move out of the blood plasma in capillaries and into surrounding tissues.
• Pressure only enough to force small molecules out of the capillaries, leaving all cells and proteins in the blood, as they’re too large to cross membranes. ‘Ultrafiltration.’

Question 54

Describe the return of tissue fluid to the circulatory system.

Answer 54

• The loss of tissue fluid from the capillaries reduces the hydrostatic pressure inside them.
• By the time blood reaches the venous end of the capillary network its hydrostatic pressure is usually lower than that of the tissue fluid outside it.
• Tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them.
• The plasma has lost water, and still contains proteins. It therefore has a lower water potential than the tissue fluid.
• As a result, water leaves the tissue by osmosis and down a water potential gradient. Tissue fluid has lost oxygen and nutrients, but gained carbon dioxide and waste.

Question 55

What happens to the tissue fluid that is not returned via capillaries?

Answer 55

The remainder of tissue fluid is returned by the lymphatic system. The large lymph vessels drain their contents back into the bloodstream via two ducts that join veins close to the heart. Contents of the lymphatic system moved by:

• hydrostatic pressure of the tissue fluid that has left the capillaries.
• contraction of body muscles squeeze lymph vessels, ensure the fluid inside them moves away from tissues in direction of the heart.

Question 56

Where is water absorbed in plants?

Answer 56

Root hair cells

Question 57

What is the vessel that transports water in plants?

Answer 57

Xylem

Question 58

What is the main force pulling water through the xylem?

Answer 58

Transpiration

Question 59

Is transpiration an active or passive process?

Answer 59

Passive- energy supplied by the sun

Question 60

How does water move out of the stomata?

Answer 60

The humidity of the atmosphere is usually less than that of the air spaces next to the stomata, creating a water potential gradient. Provided stomata are open, water vapor diffuses out of the air spaces into the surrounding air. Water lost by diffusion is replaced by water evaporating from cell walls of mesophyll cells- maintaining diffusion gradient.

Question 61

How can plants control their rate of transpiration?

Answer 61

Changing the size of stomatal pores.

Question 62

How does water movement occur through mesophyll cells?

Answer 62

Water can move via cell walls or cytoplasm of mesophyll cells.

• mesophyll cells lose water via stomata, by evaporation
• these cells now have a lower water potential and so water enters by osmosis from neighboring cells.
• the loss of water from neighbouring cells lowers their water potential.
• they then in turn, take water in from their neighbours by osmosis.

Question 63

Describe how the cohesion-tension theory explains water movement up the stem via xylem.

Answer 63

• Water evaporates from mesophyll cells, due to the sun, causing transpiration.
• Water molecules form hydrogen bonds between one another, and stick together- ‘cohesion’
• Water forms a continuous, unbroken column across mesophyll cells and down xylem.
• As water evaporates from the mesophyll cells in the leaf into the air spaces beneath stomata, more molecules are pulled up by cohesion.
• A column of water is pulled up the xylem as a result of transpiration- ‘transpiration pull’.
• Transpiration pull puts the xylem under tension, creating negative pressure within the xylem, so there is a negative pressure within the xylem, hence the name ‘cohesion tension theory’.

Question 64

Give evidence to support the cohesion tension theory

Answer 64

-Change in the diameter of tree trunks according to rate of transpiration. During the day, where transpiration is at its greatest, there’s more tension in the xylem. This pulls xylem vessel walls inwards, reducing the diameter of tree trunk.
At night, transpiration is at its lowest, less tension, trunk expands.
-When xylem vessel breaks, water does not leak out, as would be the case if under pressure. Air is drawn in, as it would under tension.

Question 65

What are the 3 pathways of water movement down a concentration gradient into the xylem?

Answer 65

• Symplast
• Apoplast
• Vacuolar

Question 66

Describe the symplast pathway

Answer 66

The symplast pathway is where water moves through tiny gaps called plasmodesmata, moving from one cell to another.

Question 67

Describe the apoplast pathway

Answer 67

The apoplast pathway is where water takes a route going from cellulose cell wall to cell wall, not entering the cytoplasm at any point. However, the apoplast pathway can only take water a certain way; near the xylem, the Casparian strip forms an impenetrable barrier to water in the cell walls, and water must move into the cytoplasm to continue.

Question 68

Describe the vacuolar pathway

Answer 68

The vacoular pathway moves molecules through the vacuoles of the plant.

Question 69

Why do plants not need complex transport systems?

Answer 69

Plants have slow rates of respiration.

Question 70

Name adaptations of the xylem

Answer 70

• Thick cellulose cell walls, strengthened by lignin, help plant support.
• Inside of the cell is hollow
• Dead cells

Question 71

How do you measure water uptake in a plant??

Answer 71

Potometer

Question 72

Define translocation

Answer 72

The process by which organic molecules and mineral ions are transported throughout the plant

Question 73

What is a source?

Answer 73

Sites of production of sugars, from photosynthesis.

Question 74

What is a sink?

Answer 74

Where sugars are stored for future use.

Question 75

What does the phloem transport?

Answer 75

Organic molecules- sucrose and amino acids.

Inorganic ions- potassium, chloride, phosphate and magnesium

Question 76

What is the mechanism of translocation?

Answer 76

Mass flow theory

Question 77

Describe the transfer of sucrose into sieve elements from photosynthesising tissue

Answer 77

• Sucrose diffuses down concentration gradient by facilitated diffusion, from photosynthesising cells into companion cells.
• Hydrogen ions are actively transported from companion cells into spaces within walls, use ATP.
• Hydrogen ions diffuse down concentration gradient through carrier proteins into sieve tube elements.
• Sucrose is transported along with hydrogen ions in co-transport.

Question 78

Describe the mass flow of sucrose through sieve elements.

Answer 78

• The sucrose produced by the source is actively transported into sieve tubes, causing the sieve tubes to have a lower water potential.
• As xylem have a much higher water potential, water moves form xylem into sieve tubes by osmosis, creating a high hydrostatic pressure within them.
• At the respiring cells (sink), sucrose is either used up in respiration or convert to starch for storage.
• These respiring cells therefore have a low sucrose content, so sucrose is actively transported into them from sieve tubes, lowering their water potential.
• Therefore water also moves into these respiring cells from sieve tubes, by osmosis.
• The hydrostatic pressure decreases in this part of the sieve tubes.
• As a result of water entering the sieve tubes at the source and leaving at the sink, there’s a high hydrostatic pressure at the source and low pressure at the sink, creating a mass flow of sucrose down the hydrostatic gradient.

Question 79

Which cells actively transport sucrose from sieve tube elements into storage/sink cells?

Answer 79

Companion cells

Question 80

Describe the structure of the phloem.

Answer 80

Phloem is made up of sieve tube elements, long thin structures arranged end to end. Their walls are perforated to form sieve plates. Companion cells associate with sieve tube elements.

Question 81

Why does the phloem contain living cells, such as companion cells?

Answer 81

They contain lots of mitochondria to produce ATP, in order to actively transport sugars and ions.

Question 82

Give evidence supporting the mass flow hypothesis

Answer 82

• Pressure within sieve tubes, shown by sap released when cut
• Concentration of sucrose higher in leaves (source) than in roots (sink)
• Downward flow of the phloem in daylight, ceases when leaves sheltered or night.

Question 83

Give evidence questioning the mass flow hypothesis

Answer 83

• Function of sieve tube plates unclear, as they would seem to hinder mass flow
• Not all solutes move at the same speed, like they should if transport by mass flow.
• Sucrose delivered at similar rate to all regions, not quicker to regions of low concentration.

Question 84

Describe ringing experiments

Answer 84

A section of the outer layer of the stem, including the phloem is removed around the circumference, while still attached to rest of plant.

Question 85

Describe the result of ringing experiments.

Answer 85

Sugars in phloem accumulate above the ring, leading to swelling. Interruption of flow of sugars below the ring and death of tissues in this region.
Concludes the phloem is responsible for sugar transport.

Question 86

Describe how radioactive isotopes can trace substances in plants

Answer 86

If a plant is grown in an atmosphere containing ^14CO2, the ^14C isotope incorporates into sugars produced in photosynthesis. Radioactive sugars can then be traced as move through plant, using audiography. Eg X-ray film blackens when exposed to ^14C radiation, corresponding to phloem tissue in stem.

Question 87

How do aphids provide evidence of sugars in phloem.

Answer 87

Aphids feed on plants, their needle like mouths penetrating the phloem, extracting the contents of sieve tubes. These contents show daily variations in sucrose content of leaves mirrored later by identical changes in sucrose content in the phloem.